Since aliphatic polyester such as polyglycolic acid and polylactic acid are degraded by microorganisms or enzymes present in the natural world such as soil and sea, they attract attention as biodegradable polymeric materials which impose little burden on the environment. The aliphatic polyesters are also utilized as medical polymeric materials for surgical sutures, artificial skins, etc. because they have degradability and absorbability in vivo.
Among the aliphatic polyesters, polyglycolic acid is excellent in gas barrier properties such as oxygen gas barrier property, carbon dioxide barrier property and water vapor barrier property and also excellent in heat resistance and mechanical strength, and so its new uses have been developed either singly or in the form of a composite with other resin materials in fields of packaging materials and the like.
An aliphatic polyester can be produced by, for example, dehydration polycondensation of an α-hydroxycarboxylic acid such as glycolic acid or lactic acid. In order to efficiently synthesize a high-molecular weight aliphatic polyester, a process comprising synthesizing a cyclic diester of an α-hydroxycarboxylic acid and subjecting the cyclic diester to ring-opening polymerization is generally adopted. For example, when glycolide, which is a cyclic diester of glycolic acid, is subjected to ring-opening polymerization, polyglycolic acid is obtained. When lactide, which is a cyclic diester of lactic acid, is subjected to ring-opening polymerization, polylactic acid is obtained.
A cyclic ester generally contains impurities such as an α-hydroxycarboxylic acid used as a raw material, free carboxylic compounds such as linear α-hydroxycarboxylic acid oligomers, and water. Since the impurities such as water adversely affect the ring-opening polymerization of the cyclic ester, it is proposed to use a cyclic ester, from which impurities have been removed, upon ring-opening polymerization.
On the other hand, an alcohol such as a higher alcohol is used as a polymerization controller of molecular weight upon the ring-opening polymerization of the cyclic ester for controlling the molecular weight of the resulting aliphatic polyester (for example, U.S. Pat. No. 3,442,871).
It is also proposed to remove impurities such as water from a cyclic ester (for example, Japanese Patent Application Laid-Open No. 301864/1996). This document indicates that the impurities such as water, an α-hydroxycarboxylic acid and low-molecular weight oligomers thereof in the cyclic ester exert various actions on an initiator, a chain-transfer agent, a catalyst inactivator and the like to inhibit the ring-opening polymerization, and so these impurities should be removed.
It is proposed to produce an aliphatic polyester by subjecting a cyclic ester having a water content of at most 80 ppm and an acid value of at most 0.10 mg KOH/g to ring-opening polymerization (for example, Japanese Patent Application Laid-Open No. 158371/1998). This document describes the fact that when the amount of water in the cyclic ester is reduced, the polymerization rate is accelerated to obtain a high-molecular weight polymer, and that when an alcohol is caused to exist in a polymerization system, the action of water can be inhibited to produce a high-quality aliphatic polyester.
In a production process of an aliphatic polyester by subjecting a cyclic ester to ring-opening polymerization, it is proposed to determine the amount of a hydroxyl group-containing compound added to a reaction system on the basis of the amount of free carboxylic compounds contained in the cyclic ester (for example, Japanese Patent No. 3075665). In this document, an α-hydroxycarboxylic acid used upon the production of the cyclic ester and linear α-hydroxycarboxylic acid oligomers are shown as the free carboxylic compounds, and the document describes that a linear saturated aliphatic monoalcohol having 12 to 18 carbon atoms is preferred as the hydroxyl group-containing compound.
This document also indicates that when the impurities such as water and free carboxylic compounds are contained in the cyclic ester, the polymerization reaction is adversely affected, and so targeting that a polymer having a targeted molecular weight is produced becomes impossible even under the same polymerization conditions. This document describes the fact that since a high water content shows a tendency to make the control of a molecular weight of the resulting aliphatic polyester difficult, the water content in the cyclic ester is preferably controlled to at most 100 ppm for the purpose of controlling the molecular weight with good accuracy.
The document further indicates that although water in the cyclic ester can be easily removed in a purification and drying process just before the polymerization, the free carboxylic compounds are difficult to be removed, exert a great influence on the polymerization reaction, and moreover the cyclic ester is ring-opened by a minute amount of water during its storage to easily form new free carboxylic compounds. In this document, it is proposed to determine free carboxylic compounds contained in the cyclic ester and add a hydroxyl group-containing compound (for example, higher alcohol) in an amount corresponding to this amount, thereby producing an aliphatic polyester having a targeted molecular weight.
As described above, the fact that a cyclic ester, from which impurities such as water and free carboxylic compounds have been removed, is used in a process for producing an aliphatic polyester by ring-opening polymerization of the cyclic ester, and an alcohol, particularly a higher alcohol is used as a molecular weight modifier was the state of the art. A technique of producing a cyclic ester such as glycolide with high purity has been developed in recent years, and so it has been comparatively easy to produce or obtain a cyclic ester containing impurities such as water in little amount. On the other hand, it is expected that a problem attended on the use of a higher alcohol as a molecular weight modifier is potentialized under the circumstances that aliphatic polyesters such as polyglycolic acid are produced on an industrial scale.
The reason why alcohols are used as a molecular weight modifier is that the hydroxyl group thereof contributes to the control of the molecular weight. The reason why a higher alcohol among the alcohols is generally used is that the concentration of the hydroxyl group in the higher alcohol is low, and so a weighing error becomes little to gain an advantage from the viewpoint of a charging operation upon polymerization. For example, when an aliphatic polyester such as polyglycolic acid is produced on a small scale, the amount of an alcohol used as a molecular weight modifier becomes greater as the molecular weight of the alcohol increases though the amount thereof is extremely small, so that weighing accuracy and handling characteristics are improved. In addition, since the higher alcohol has a boiling point higher than a temperature of ring-opening polymerization, and so a stable polymerization operation can be expected.
However, a higher alcohol such as lauryl alcohol (also referred to as “dodecanol” or “dodecyl alcohol”) is expensive and has involved a problem that since it is viscous, it tends to remain in a charging device (for example, syringe) when charged into a polymerization reaction system, and so its loss becomes great. In addition, when the higher alcohol is used in the production of an aliphatic polyester in an industrial scale, it involves such many problems related to increase in the production cost of the aliphatic polyester that (1) its amount used becomes great, (2) a tank and a weighing device, which are temporarily used for charging the alcohol into a polymerization reactor, are enlarged in size, (3) an apparatus used is required to take safety measures because the alcohol is combustible, (4) cleaning of the apparatus is required after the apparatus are used, and (5) a cleaning liquid used in the cleaning of the apparatus must be treated.
In addition, when a higher alcohol is added to a cyclic ester, an aliphatic polyester having a polymer structure that the higher alcohol is introduced therein is formed because the higher alcohol also acts as an initiator, and so the physical properties of the resulting aliphatic polyester are changed. Further, the higher alcohol is insufficient in solubility in the cyclic ester, and consequently a ring-opening polymerization reaction has taken place unevenly to cause a problem in the precise control of the molecular weight and melt viscosity of the resulting polymer.